Injection moulding apparatuses often comprise needle valve nozzles, with which a flowable mass at a predeterminable temperature and under high pressure is supplied to a removable mould insert. The needle valve nozzles have pneumatically, hydraulically or electrically operated closing needles, which periodically open and close the gate openings in the mould insert. This enables the most precise possible material dispensing, especially with a rapid shot sequence. However, the flowable mass can also be injected in segmented manner, e.g., during cascade moulding, by a definitely controlled opening and closing of the injection point.
Each closing needle is mounted so as to move axially in the die region of the injection moulding apparatus and it is preferably led centrally through a flow channel for the moulding mass in the nozzle region (for this, see for example DE 3 249 486 C3 or DE 3 403 603 A1). The flow channel usually terminates in a nozzle mouthpiece, forming a nozzle exit opening at the end face. In the closing direction, the lower end of the closing needle engages in a seal seat fashioned in the nozzle mouthpiece or in the mould insert.
For the tight guidance of the closing needle, a guide bushing or sealing sleeve is customarily inserted in the distributor plate of the injection moulding apparatus, which receives the cylindrical shaft of the closing needle (for this, see for example DE 39 26 357 A1 or EP 1 223 020 B1). Between the closing needle and the guide bushing there remains a free space in the form of a hollow cylinder, into which flowable material enters during the operation of the injection moulding apparatus, so that the needle is sealed off against the flow channel. At the same time, a lubricating effect is produced, which decreases the friction between the closing needle and the bushing.
Moreover, it is known from DE 20 2006 017 380 U1 how to provide at least one guiding portion in a guide bushing for the closing needle of a needle valve nozzle, which encloses the closing needle with slight movement play, as well as at least two expansion portions, whose diameter is larger than that of the at least one guiding portion. The expansion portions serve as receiving spaces for the flowable material in the through-opening. By using a plurality of such expansion portions, on the one hand a lubricating function for the closing needle can be assured by the flowable material collected in the receiving spaces, while at the same time the cumulative sealing effect of the expansion portions filled with flowable material is improved.
Despite optimal design of such a sealing system, on account of the relative high pressures within the die and on account of the travel movements of the needle, as well as the low-viscosity materials used, it cannot be prevented that the moulding mass will penetrate through the guiding and sealing bushing to the outside. Material losses are the result. Furthermore, the material residues foul both the closing needle and the die, which not only affects the sealing action, but also may over time hinder the opening and closing movement of the closing needle. Costly cleaning or servicing work is unavoidable.